Cheapest to most expensive:
Home schooling with own children or closely related/allied (requires parent with sufficient educational capability) – no transport time, no additional facility costs, no need to send kids home or arrange special logistics. Only outside costs are standardized testing and child welfare monitoring/followups.
Open air school (requires temperate climate) – saves money on the facility upkeep, no need to hire significant numbers of janitors, etc. but you may need year-round schooling depending on the number of good instruction days.
Single room school – saves money on gyms, etc. but of course you don’t have those amenities. Works much better in temperate climates.
Conventional daytime school
(For completeness: Boarding school (that is, the cost burden of whatever other schooling type, plus boarding and parenting))
One-on-one or similar small staffing ratio tutoring
Best quality to worst quality, with the usual caveat that different children do better in different settings:
One-on-one or similar small staffing ratio tutoring (if the parent is not capable of expert instruction at that level)
Home schooling with own children or closely related/allied (requires parent with sufficient educational capability)
Conventional daytime school (with highly supportive parents), with the better performance being in the 10-15 student range, with increasingly poor results as class size increases to 30 students, where personal coaching and interventions become an sliver of class time
Single room school
Open air school